Conventionally, various planer type pressure sensors are proposed which detect a load applied on a support member, such as a bed, a mattress or a seat and judge whether a human, an animal or an object exists thereon or not. Some are proposed which detect minute vibration generated by a human heartbeat or breath in addition to the object existence judgment. The following patent document 1 illustrates an invention relating to such load-detecting device. The load detecting device of the patent document 1 includes a vibration detecting means which is provided in a support member supporting a human body and which includes a vibration generating portion and a vibration detecting portion provided adjacent to the vibration generating portion. The load-detecting device of this document includes a vibration characteristic calculation means and a load calculation means. The vibration characteristic calculation means calculates vibration characteristic of the vibration transmitted from the vibration-generating portion based on the output signal from the vibration-detecting portion. The load calculation means calculates the load applied to the vibration detecting means based on the output signal from the vibration characteristic calculation means. Further, the load-detecting device includes a human body extracting means, which extracts vibration component of the supported human body from the output signal from the vibration-detecting portion. This extracting means extracts vibration component such as heartbeat or aspiration (breath).
Patent Document 1 (JP2000-230853A, paragraph 8 to 16, FIG. 1 to 5, 8 to 10)
When a human body is supported by a supporting member a load constantly generated thereby is called a static load and the vibration component of the load extracted from the human body vibration extracting portion based on the heartbeat or aspiration is called as a dynamic load generated suddenly Since the vibration of this dynamic load is very small and the output thereof is not so large. In order to easily detect such small vibration, a piezoelectric element is used as the vibration-detecting portion (see patent document 1 above) to enlarge the sensing area. If such enlargement of the sensing area is performed, some foreign (other than human body) noises may be detected.
A flexible material such as polyurethane material is normally used for the supporting member for supporting human body, considering the comfort issue. When the piezoelectric element is arranged between the supporting member and the human body, the piezoelectric element may be deformed undesirably for detecting the small vibration due to the static load generated by the human body. This deformation may cause output performance of the piezoelectric element or S/N ratio. Particularly, since the heartbeat vibration signal is very small, the output performance and S/N ratio may drop to deteriorate the detection accuracy.
Accordingly, this invention pertains to improve the detection accuracy especially to improve the detection of a load displacement caused by a human body supported on a flexible supporting member or the vibration generated by the human body.